The present invention relates to AC-DC and DC-DC converters, voltage adapters, battery chargers and similar circuits which may or may not be functioning in a switched mode.
In voltage adapters, battery chargers, and power supplies that can be directly plugged into an AC power source, and more generally into a DC-DC converter, with or without isolation, it is necessary to provide protection systems from short-circuits, over-voltages, over-temperatures, etc. The protection systems are necessary to preserve the integrity of the integrated device that commonly contains power supply, drive and control circuitry of the converter, and/or of any other external component that must be protected. These other external components include power transistors, transformers, and diodes, for example. Such protection is commonly provided by connecting external components to dedicated pins of the integrated circuit of the converter.
Thermal protection is commonly provided by an active or passive component that is sensitive to temperature. This component is suitably connected to one or more fixed or variable resistors, or to another passive or active element that biases an external temperature sensing line coupled to two dedicated pins of the integrated circuit. On a first pin is available a constant reference voltage that can be generated by an integrated regulator to which a terminal of the external line sensitive to the temperature is connected. Another functional node of the sensing line is coupled to a second input pin of the integrated circuit to force the disabling of the power switch or switches of the converter, and thus to turn-off completely the converter when a certain maximum temperature is surpassed.
A drawback of these known systems is the inability of self-recovering when the temperature returns below the threshold value. This is because the internally generated biasing voltage of the network detecting the temperature becomes null upon the turning-off of the entire integrated device, thus placing the system in a latch condition. To restart, the system must be momentarily disconnected from the power source.
In applications where this is unacceptable or inconvenient, an auxiliary power supply or voltage regulator is used for generating a constant reference voltage for biasing the sensing temperature line. This approach is burdensome because of the extra cost of the auxiliary supply circuit, and it is in contrast with recent European Community rules on the limitation of energy absorption from the power source under no load conditions, for example.
A diagram of a typical power supply that can be directly connected to a power source and employing a DC-DC converter with a transformer T1 provided with an auxiliary winding L1 for self-supplying the integrated circuit IC during normal operation of the converter is depicted in FIG. 1. After the circuit has been started, the voltage induced on the auxiliary winding L1 is rectified by the diode D1 and filtered by the capacitor C1, thus providing a certain self-supplying voltage Vcc to the integrated circuit IC of the converter.
Start-up takes place customarily by way of a start-up resistance Rstart-up or an equivalent active component that can be, as in the depicted example, directly coupled to the power source, or to the output node of the rectifying bridge. Therefore, on the Vcc node a suitably limited voltage is produced by way of the voltage divider of which Rstart-up is part and through a voltage regulator circuit, which is usually internal to the integrated circuit IC. The functional circuits of the integrated circuit are powered at a regulated supply voltage enabling and driving the switching of the power switch PW at a certain oscillating frequency starting up the DC-DC converter. Upon the start of the oscillations, the voltage induced on the self-supply winding L1 is the voltage Vcc. This voltage continues to power at a suitable level the IC, thus ensuring at steady state conditions a correct functioning of the IC.
Two typical protection networks are depicted in the diagram of FIG. 1. Protection from over voltages is implemented by the voltage divider RA and RB dimensioned such to disable the converter circuit when a certain maximum threshold voltage is surpassed (DISABLE signal). In order to restart the converter it is necessary to unplug it from the power source and reconnect it.
As shown, the thermal protection line may be typically formed by an active or passive component sensitive to the temperature. In the example shown, the component includes a nonlinear resistor with a negative temperature coefficient (NTC) and a series connected resistor Rs. A second resistor can optionally be connected in series between the NTC and Rs to make it easier to set a desired threshold temperature.
As discussed above, the temperature detection line (NTC and Rs) is usually biased at an appropriate reference voltage that is internally generated by connecting the line to a dedicated pin, Vref, of the integrated circuit IC. Alternatively, where it is necessary to ensure a self-recovery function, the temperature sensing line is biased by an auxiliary regulator AUX, illustrated with dashed lines, such to allow the self-recovery of an operative state by the system upon the returning of the temperature below the fixed threshold value.
The stopping of the converter in presence of an excessive temperature is ensured by suitably coupling the temperature detection line to a dedicated input pin A-REC of the integrated circuit IC. This is done so that when the temperature threshold is surpassed any further switching of the power switch PW, is disabled. In case of other configurations of the converter employing two or more switches, they are also disabled. This practically disables the whole integrated circuit. It is evident that there is a need for a thermal protection system with a self-recovery ability without requiring the use of an external auxiliary voltage regulator for biasing the external temperature sensing line.
There are numerous applications, as alarm and security or safety systems, whether remote or not, wherein it is required to switch off completely the power supply to the functional circuits of the particular system or subsystems thereof. This is done whenever a certain pre-established threshold of a physical quantity is typically represented in general terms by the change of the state of an ON/OFF type sensor. The physical quantity includes temperature, and even other levels such as luminance, sound level, pressure, liquid level, presence of smoke and the like. Even in these applications, the ability of self-recovering of the power supply by re-enabling the functioning of the converter when the alarm condition is no longer present can be necessary or advantageous.
It is an object of the present invention to provide a self-disabling and self-recovering converter that uses a specific sensor for monitoring when an electrical or physical quantity reaches a certain threshold value within a power supply. This sensor advantageously uses as a biasing voltage the same regulated self-powering voltage during the start-up phase of the converter circuit.
In this way it is possible to ensure the availability of a voltage reference that is always present through the start-up circuit which is directly connected to the power source or rectifying bridge output, even while the integrated circuit of the converter is latched because of the switching of the sensor.
The self-recovery function of the disabling circuit is ensured even though the converter circuit has been turned off, which brings the entire power supply to very low consumption conditions. These conditions are practically equivalent to the current consumption of the start-up circuit of the converter, plus the current consumption of the sensor line can be made very low.
The disabling system with self-recovery ability according to the present invention comprises a regulator stage which is commonly intrinsic to the start-up circuit of the device, and a modifiable voltage clamp chain that may be customarily formed with a plurality of Zener diodes or equivalent devices connected between the output node Vcc of the self-supply rectifying circuit and a common ground node of the circuit.
The disabling system further includes a voltage divider of the regulated self-supply voltage, and a hysteresis comparator having an inverting input coupled to a first node of the voltage divider, and a second non-inverting input functionally connected to a pin of the IC device to which is coupled a functional node of an external sensing line of the physical or electrical quantity being monitored. The output of the comparator controls a first hysteresis transistor having its current terminals coupled to a second node of the voltage divider and to the common ground node, and a second transistor sectioning the clamp line by short-circuiting to ground when turned on a part of the Zener diodes or functionally equivalent devices that form the clamp chain.
A logic AND gate includes an input functionally coupled through an inverting stage to the output of the hysteresis comparator, and an output generates a disabling logic signal for all functional circuits of the comparator. The external sensor line is biased by connecting it through a dedicated pin of the integrated circuit of the comparator to the regulated self-supply voltage.